CMOS technology innovations over the last decades open the door to the possibility of designing full CMOS integrated systems at THz frequencies. The small antenna size at THz frequencies makes CMOS and silicon attractive for steerable 2D transmitter and receiver arrays. Previous works successfully showed THz source arrays with the use of on-chip antennas [4, 10-13]. However, it is still a challenge implementing such arrays that are frequency and phase locked, with significant radiated power and efficiency in standard CMOS without costly additions. The solution which will be described below aims to solve several existing issues in CMOS THz radiating arrays, for example:
The first issue is antenna efficiency, as is and in an antenna's array. The silicon bulk in standard CMOS has a high permittivity and low resistivity that incurs significant radiation loss at 0.3 THz with bulk thickness of 100-800 μm. If an on-chip antenna is designed upon a grounded silicon substrate (low or high relative permittivity), electromagnetic waves will radiate and propagate out of the antenna in all directions. All the waves radiating inside the substrate will reverberate between the ground-plane and the dielectric/air interface. After few ricochets, the angle of reflection will increase and after a critical angle, the waves will get trapped within the substrate and get coupled to surface waves modes (TE0 and TM1 being the main contributor. Few solutions were developed to circumvent this problem (such as Superstrate Quartz layer, Backside silicon lens), however, they all have a cost, require an external element and complicate the integration of the chip.
Namely, previous works added costly lens with backside radiation [11-12] or a quartz superstrate [13] to keep a reasonable efficiency. Antenna efficiency becomes even more of a challenge for 2D arrays on-chip, since it degrades with the silicon total area, thus making it not scalable. It is also known that antennas' array systems require a large silicon area (being non-proportionally larger than an area required by a single antenna) in order to achieve its maximum gain.
The second challenge is efficient THz generation in each element of such an array. In that sense, a Voltage Controlled Oscillator (VCO) is usually preferable to a multiplier chain due to the DC power consumed especially in power amplifiers. In the ubiquitous cross-coupled pair, there is inevitable tradeoff between its tuning range, output power and phase noise, especially when additional buffers and passive multipliers are used to drive the antenna at harmonics of the generated fundamental.
The third challenge is locking the array. As known in the art, active multiplier chains can be used to lock a signal to a low frequency reference at the cost of significant DC power and area. Individual PLLs can also be used, again with area and power overhead, but also with output power loss due to loading. Subharmonic injection locking can be area and power efficient, but still requires lower (but mm-wave) LO distribution that consumes area, power and do not scale in 2D so easily. It should be noted that locking of THz radiating sources/arrays is a specific technical problem, which is especially acute in the so-called “THz Gap” (0.3 THz-3 THz).
Much effort has been put in recent years to cover the ‘THz Gap’ by extending the operation range of integrated circuits from millimeter waves toward that THz range (300 GHz-3 THz). It should be noted that above the THz gap, the problem is not so acute since frequencies higher than 3 THz can be served by optical equipment. THz signals are desirable for many applications, but they are limited by their jitter (or phase noise), meaning that their frequency and amplitude are not constant over time. Moreover, current ways for achieving locked THz sources require design complexity, especially when employed in phased-array systems. This is due to several issues, such as: i) LO power distribution loss (splitter) signals (due to the high frequencies), ii) Complexity of Phased-Locked Loops (PLL) and iii) Very high DC power consumption of Active Multiplying Chains (AMC) at such frequencies. Currently, these are overcome at the cost of using expensive technologies, e.g. InP and GaN. Even though THz signal generation and detection have already been demonstrated in a relatively cheap CMOS technology (i.e., in Complementary Metal-Oxide-Semiconductor technology for constructing integrated circuits), various challenges still prevent CMOS technology to become practical for such applications. One important problem is that current CMOS transistors hardly demonstrate any power gain at the THz range beginning from 0.3 Thz (300 GHz, so-called fmax, varies in different CMOS versions). Due to that, signal generation has to rely on harmonic generation of lower frequency (fundamental frequency) sources/transmitters, typically in the mm-wave range. The main challenges in that approach are to achieve high output power, high generation efficiency, high on-chip radiation efficiency and good rejection of the fundamental frequency and other unnecessary harmonics.
Previously known solutions generated J-band (220-325 GHz) signals by using the second harmonic of a D-band frequency source [1], or the third harmonic of a W-band source [2-5]. In the mentioned works, in general, output of the source (usually of an oscillator such as a Voltage Controlled Oscillator VCO) is coupled by a transformer to an antenna, the transformer introducing additional loss but also radiating energy, therefore decreasing the power injected into the antenna. The antenna challenge in that case is not only to radiate efficiently the required harmonic from the interfering silicon substrate of CMOS, but also to reject the fundamental frequency, thus further improving the radiated signal quality and the overall power efficiency. One improvement was demonstrated in [2], where an on-chip loop antenna was directly connected to the VCO transistors' drains. However, to make the on-chip THz radiating source practical, e.g. usable for coherent communication or radar imaging, the frequency and phase of the source need to be locked to a reference source.
One approach is to use a PLL (Phase Locking Loop) at the fundamental [3] frequency, which drastically increases the circuit complexity, cost and DC consumption. Moreover, in case there is an array of radiating sources (VCOs) that approach would require either an individual PLL per element, or a lossy, complicated distribution network for mm-wave or even THz signals.
Another approach known in the prior art was to combine and couple numerous VCOs on the same chip running in parallel and employing mutual locking [2]. In such an approach, however, the VCOs outputs have to be power-combined before the antenna or to provide antennas within each element (VCO) area. The power combination is then carried at the fundamental frequency, which is a disadvantage. Moreover, mutual locking does not necessarily suffice for achieving a locked transmit signal from the overall array; this issue is sometimes dealt with by placing a few synthesizers within the array, in order to injection-lock the overall array, and this solution is again costly, complex and DC consuming. Signal distribution is then done at lower, less lossy, frequencies but the cost in silicon area and the DC power consumption is higher.
A number of publications on the above-discussed subject matter are mentioned in the section “References” presented after the description.